These studies were very empirical, and they really were aimed entirely at determining how much to charge middle-aged white people for insurance. They did a good job of that, but they didn’t account for any number of confounding factors. However, once they had published the tables, these tables went from being essentially descriptive — “middle aged people seem to live longest in these height/weight ratios” — to be taken as prescriptive — “everyone’s ideal weight is given by these tables of height/weight ratio.” Now we define these “ideal weights” in terms of body-mass index, BMI, which sounds much more precise and scientific, but turns out to be simply a height/weight ratio.

Pretty much anyone can see that BMI is questionable — for example, a champion bodybuilder with a competition body fat of 3 percent may well have a “very obese” BMI. (On the other hand, it’s unclear that very low body fat is necessarily healthy either — in fact, we know it’s certainly not healthy for women.)

The problem is that epidemiology, the study of health and disease across large populations, keeps finding results that don’t quite fit this idea of ideal weight. Most recently, a study by Katherine Flegel and others published in January of this year showed that the notion of ideal weight was massively oversimplified. The study found two things: first, that for younger people, BMI doesn’t have any strong effects, and as you get older, the BMI associated with the least likelihood of dying increases.

In other words, if you don’t want to die the data suggests you actually want a slightly higher BMI as you get older.

What’s more, other studies say that BMI isn’t as good a predictor as simply the length of your belt — the larger your waistline, the more likely you were to have a whole lot of different health problems like type-2 diabetes. (This one does fit the bodybuilder example, too — bodybuilders do generally have small waists.)

Now, this can be taken too far — there’s no doubt that real obesity has bad effects on your health. (My knees would tell you that, if asked. And if knees could talk.) But the truth is that being a little overweight is either not harmful or may actually be helpful.

Myth #2: Your Weight Depends Only On Calories-In Versus Calories-Out.

This one has also come up here before. My own data, collected over more than six months and my first two 13-weeks experiments, has shown that it certainly doesn’t hold for me. Over the first about 10 weeks of my very low-carb diet, I lost weight much more rapidly than could be accounted for by a calorie deficit; since then as I crossed the Great Plateau, my weight loss has been considerably less than calorie deficit should predict.

As I've posted before, I'm completely with you on virtually all diet points and personal experience. I only need to lose maybe ten to twenty pounds, but those pounds do NOT want to go unless I cut my calories below 1,000, then I can lose a couple of pounds fairly quickly, the problem being they come BACK even more quickly.

It is probably age, and probably boosting the aging factors like testosterone and thyroid would help. I also used to get way more exercise than my aging body will now stand for, others older than me are luckier in those regards, but even four hours walking does not impress my metabolism at all today, and I think it did twenty years ago.

One other thing, 3600mg of fish oil sounds like rather a lot (especially if you eat some actual fish once or twice a week not to mention other n-3 rich or boosted foods), if you need that much you are probably getting more n-6 than you need as well.

You called your diet low carb, high fat, and high protein. The correct low carb diet is moderate protein. You are probably aware of the Jimmy Moore blog. A year ago he discovered his low carb diet was too high in protein; the excess protein was being metabolized like carbs.

A guy called Mike Geary has some interesting ideas about health, fitness and so on. No, I'm not a 'marketing associate' of his, but he has a web site with some interesting theories. He's an exercise freak, lives in Colorado. Might be interesting for you to get in touch with him.

It would muddy the waters of the current thirteen week experiment but you might want to try some fish oil supplementation. Neither Gary Taubes nor Tim Ferris spend much time on the omega3/omega 6 imbalance that is a consequence of the graincentric American diet but there are Paleo and Primal people who focus on it as a major metabolic sticking point.

Pretty much the same experience here. I'm 70, 6'2' and have a 43 inch waist. I'm down from 215 dropping agricultural grains and plateaued at 189. I go up to about 193 if I get my exercise disrupted by things like a cold then back down to 189, but no further. Like you I am using the 5:2 diet - 2 non consecutive days semi fast of 600 calories. Too soon to tell if i can break the 189 barrier. Michael Moseley's BBC/PBS doco Eat, Fast, Live breaks out the research on various forms of fasting including 5:2. Good luck breaking the barrier.

Charlie, You might need to take a vitamin/mineral complex to help with weight loss. Trace elements like Chromium and Selenium affect blood sugar and weight loss -- and Americans are chronically short on both.

I've used the low-glycemic diets off and on for a decade with good success. In my experience, occasionally you will get stuck, but if you stick with it, the decline will eventually resume. You're doing the right thing - add some moderate glycemic carbs, like beans, for a couple of weeks, and then go back to the stricter version. That way, the stall is more bearable.

I would not introduce high-glycemic carbs once a week. They cause no long term damage, but, again in my experience, every time I do that, I get a stall of three or four days in the downtrend.

A pound and a half a week is great, and just what you want, if you want to keep the weight off. After all, if you are looking for a 50 lb loss, that's only eight months!

Are you doing any exercise? I think in one of your previous articles you mentioned martial arts and weightlifting. For the longest time I had trouble fitting in a substantial daily weightlifting session just because of my work schedule, but I started doing 20 minutes in the morning and 20 minutes plus a little extra after work. I would just use a couple dumbbells in front of the TV so I didn't have to head out to the gym. Even though it wasn't much, it was still a big help.

Just by the way, my weight today was 269, which means I'm actually sustaining something close to 2 pounds a week for two weeks now. (Actual slope of the linear fit is -0.248 lb/day). Also, the standard deviation is down from last experiment -- 1.96 vs 2.3 -- and this is now around 3 sigmas down from the start weight. Still too little data to draw a conclusion but it begins to look significant.